The present invention is concerned with an electrode activating compound for a gas discharge tube containing a metal oxide selected from a metal of the fourth subgroup of the periodic system.
An electrode activating compound, which utilizes a metal oxide from the fourth subgroup of the periodic table, for the electrodes of a voltage overload arrester or a surge voltage arrester is disclosed in U.S. Pat. No. 3,691,428 which was based on German O.S. No. 19 35 734. In this U.S. patent, an electrode activating compound included thorium oxide.
Electrode activating compounds are generally used in the case of gas discharge tubes and according to the purpose of the application are composed to significantly influence the important electric characteristic quantities in each case. Where the gas discharge tube is utilized as a surge voltage arrester or is used as a controllable switching tube, which is known as a cold cathode thyratron, or is used as a flash tube, an important requirement is directed towards a low glow-arc transition. This is defined as a momentary current value in the case of which the fired gas discharge crosses over from a glow discharge into an arc discharge. Low values produce a good firing behavior, above all, in the case of firing via a third electrode with low firing currents, and by means of good current carrying capacity, they produce a long service life. Beyond this, the minimum operating voltage is advantageously of a small value.
A low glow-arc transition can be attained with an electrode activating compound which, among other things, contains a radioactive thorium oxide as an effective constituent. The security measures, which are necessary because of the radioactivity and therefore the danger to the environment, as well as the considerable waste disposal costs, provide a serious disadvantage to the use of thorium oxide as an electrode activating compound.